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Douglas Harper's Etymology Dictionary

bi-

word-forming element meaning "two, twice, double, doubly, once every two," etc., from Latin bi- "twice, double," from Old Latin dvi- (cognate with Sanskrit dvi-, Greek di-, Old English twi- "twice, double"), from PIE root *dwo- "two." Nativized from 16c. Occasionally bin- before vowels; this form originated in French, not Latin, and might be partly based on or influenced by Latin bini "twofold" (see binary).

The Collaborative International Dictionary

bi-

Deuto- \Deu"to-\or Deut- \Deut-\ (d[=u]t-)[Contr. from Gr. dey`teros second.] (Chem.) A prefix which formerly properly indicated the second in a regular series of compound in the series, and not to its composition, but which is now generally employed in the same sense as bi- or di-, although little used.

Wiktionary

bi-

pre. 1 two 2 (context chemistry proscribed English) half

Longman Dictionary of Contemporary English

bi-

prefix
EXAMPLES FROM OTHER ENTRIES
▪ a biannual meeting